1 files changed, 41 insertions, 13 deletions

diff --git a/src/mongo/s/commands_public.cpp b/src/mongo/s/commands_public.cpp
index f2d3789c2a6..7a5a8281717 100644
--- a/src/mongo/s/commands_public.cpp
+++ b/src/mongo/s/commands_public.cpp
@@ -2525,20 +2525,36 @@ bool PipelineCommand::run(OperationContext* txn,
         return aggPassthrough(conf, cmdObj, result, options);
 
     /* split the pipeline into pieces for mongods and this mongos */
-    intrusive_ptr<Pipeline> pShardPipeline(pPipeline->splitForSharded());
+    BSONObj firstMatchQuery = pPipeline->getInitialQuery();
+    ChunkManagerPtr chunkMgr = conf->getChunkManager(fullns);
+    StatusWith<BSONObj> swShardKeyMatches =
+        chunkMgr->getShardKeyPattern().extractShardKeyFromQuery(firstMatchQuery);
+    uassertStatusOK(swShardKeyMatches.getStatus());
+    const BSONObj& shardKeyMatches = swShardKeyMatches.getValue();
+
+    // Don't need to split pipeline if the first $match is an exact match on shard key, unless
+    // there is a stage that needs to be run on the primary shard.
+    const bool needSplit = shardKeyMatches.isEmpty() || pPipeline->hasOutStage();
+    intrusive_ptr<Pipeline> pShardPipeline(needSplit ? pPipeline->splitForSharded() : pPipeline);
 
     // create the command for the shards
     MutableDocument commandBuilder(pShardPipeline->serialize());
-    commandBuilder.setField("fromRouter", Value(true));  // this means produce output to be merged
+    if (needSplit) {
+        // this means produce output to be merged
+        commandBuilder.setField("fromRouter", Value(true));
 
-    if (cmdObj.hasField("$queryOptions")) {
-        commandBuilder.setField("$queryOptions", Value(cmdObj["$queryOptions"]));
+        if (!pPipeline->isExplain()) {
+            // "cursor" is ignored by 2.6 shards when doing explain, but including it leads to a
+            // worse error message when talking to 2.4 shards.
+            commandBuilder.setField("cursor", Value(DOC("batchSize" << 0)));
+        }
+    } else {
+        // If we don't need to split, just send the command directly to the shard, as is.
+        commandBuilder.setField("cursor", Value(cmdObj["cursor"]));
     }
 
-    if (!pPipeline->isExplain()) {
-        // "cursor" is ignored by 2.6 shards when doing explain, but including it leads to a
-        // worse error message when talking to 2.4 shards.
-        commandBuilder.setField("cursor", Value(DOC("batchSize" << 0)));
+    if (cmdObj.hasField("$queryOptions")) {
+        commandBuilder.setField("$queryOptions", Value(cmdObj["$queryOptions"]));
     }
 
     if (cmdObj.hasField(LiteParsedQuery::cmdOptionMaxTimeMS)) {
@@ -2547,20 +2563,23 @@ bool PipelineCommand::run(OperationContext* txn,
     }
 
     BSONObj shardedCommand = commandBuilder.freeze().toBson();
-    BSONObj shardQuery = pShardPipeline->getInitialQuery();
 
     // Run the command on the shards
     // TODO need to make sure cursors are killed if a retry is needed
     vector<Strategy::CommandResult> shardResults;
-    STRATEGY->commandOp(dbName, shardedCommand, options, fullns, shardQuery, &shardResults);
+    STRATEGY->commandOp(dbName, shardedCommand, options, fullns, firstMatchQuery, &shardResults);
 
     if (pPipeline->isExplain()) {
         // This must be checked before we start modifying result.
         uassertAllShardsSupportExplain(shardResults);
 
-        result << "splitPipeline"
-               << DOC("shardsPart" << pShardPipeline->writeExplainOps() << "mergerPart"
-                                   << pPipeline->writeExplainOps());
+        if (needSplit) {
+            result << "splitPipeline"
+                   << DOC("shardsPart" << pShardPipeline->writeExplainOps() << "mergerPart"
+                                       << pPipeline->writeExplainOps());
+        } else {
+            result << "splitPipeline" << BSONNULL;
+        }
 
         BSONObjBuilder shardExplains(result.subobjStart("shards"));
         for (size_t i = 0; i < shardResults.size(); i++) {
@@ -2572,6 +2591,15 @@ bool PipelineCommand::run(OperationContext* txn,
         return true;
     }
 
+    if (!needSplit) {
+        invariant(shardResults.size() == 1);
+        invariant(shardResults[0].target.getServers().size() == 1);
+        const BSONObj reply = shardResults[0].result;
+        storePossibleCursor(shardResults[0].target.toString(), reply);
+        result.appendElements(reply);
+        return reply["ok"].trueValue();
+    }
+
     if (doAnyShardsNotSupportCursors(shardResults)) {
         killAllCursors(shardResults);
         noCursorFallback(pShardPipeline, pPipeline, dbName, fullns, options, cmdObj, result);